Conventionally, in electrical products such as mobile phones, digital cameras, digital video cameras, and remote controllers, a housing containing an audio device such as a microphone and a speaker is provided with an opening for allowing transmission of sounds, and a waterproof sound-transmitting membrane is used to cover the opening. As an example of such a waterproof sound-transmitting membrane, Patent Literature 1 describes a porous membrane obtained by stretching a sheet made of polytetrafluoroethylene (PTFE).